Design idea: IC controls room temperature

When the temperature in a room rises to a certain level, it may become an uncomfortable place to be. A fan can help, but we do not want the fan on at all times, only when the temperature rises above a particular threshold. By building a simple circuit with an op amp, a comparator, and a voltage reference in one IC package, a simple temperature-controlled fan circuit can be built with minimal external component count.

Measurement set-upA temperature-controlled fan application circuit as shown in Figure 1 has the TS12011 IC powered from a 2.5V voltage from a buck converter that can also be used to drive other ICs such as a microcontroller.

Click on image to enlarge.

The buck converter input voltage is 6V and is generated by four 1.5V AA batteries. Furthermore, the batteries power any LM35 temperature sensor and a silicon-controlled rectifier (SCR) to control fan operation. The temperature sensor provides an output voltage that is linearly proportional to ambient temperature in Celsius. For every one (1) degree Celsius increase/decrease in temperature, the temperature sensor’s output voltage increases/decreases by 10mV.

In this application circuit, the fan is set to turn on when the temperature rises to 26.5°C. For instance, at a temperature of 26.5°C, the temperature sensor will generate an output voltage of 265mV that will in turn generate a HIGH state at the output of the comparator and the FAN will turn on. For testing purposes, an external power supply was substituted for the temperature sensors. Figure 2 shows the complete circuit where a demo board is used along with the IC.

Click on image to enlarge.

ResultsWhen the power supply voltage applied to the input of the op amp reaches approximately 265mV, the fan turns on successfully. A 265mV voltage corresponds to a temperature of 26.5°C.

Considerations In this IC, the LHDET pin is useful for capturing one-time events. While this function was not explicitly used in this application, the LHDET pin was connected to VIN thereby disabling one-time-latch operation. For a detailed explanation of this function, please refer to the video referenced below.

ConclusionIn summary, the application circuit described herein achieves a three primary goals: a) reducing the total number of analog ICs used from 4 to 2, thereby saving significant pcb area; b) low-supply-voltage operation from 0.8V to 2.5V; and c) reducing the supply current consumed by the op amp/reference/comparator functions to 1.5µA.